Volunteering is defined by its charitable purpose for a public good, not by the specific skill used.
Let me try an analogy:
A chef who cooks a free meal for a homeless shelter is volunteering. That same chef publishing a recipe online or making a cooking tutorial is sharing knowledge, not volunteering. The act of 'cooking' or 'publishing' is neutral. It becomes volunteering only when the primary, direct, and organised purpose is to serve a charitable cause without expectation of personal gain.
Disclaimer: I have been consistently doing a lot of open source in the last 10 years. I would consider none of that as volunteering.
So also things like helping kids with their homework or giving people courses in your hackerspace, repaircafes, reading with others can fall into that.
So while maybe not all software that is open source also is automatically useful for the commons as it is now the definition is way too narrow. If you write software that helps one of the existing recognized causes it is openns source. If you write an open source photoshop or spend days working on software that keeps the world running you don't. But we need the latter people and supporting the former people makes the world a better place.
I'm guessing it doesn't count if you are being helped to help kids or give courses, does it? So not only it depends on what it is, it also depends on how it is done.
Open source in itself is not charitable, and many people get paid to contribute to open source projects.
My point is that I agree that some open source projects can count as volunteering, just like some masonry work. But I wouldn't say that "open source" should count as volunteering, just like for masonry.
also the term "gemeinnützig" is about the end result, not how it is produced. FOSS is gemeinnützig, even if the producers are paid.